Start by looking at each word, individually, in the first few pages. Highlight, or note each word that is, or could be, scary or unknown.

Let's just look at the first paragraph, we'll add to readerofbook's answer above:

The boy with fair hair lowered himself down the last few feet of rock and began to pick his way toward the lagoon. Though he had taken off his school sweater and trailed it now from one hand, his grey shirt stuck to him and his hair was plastered to his forehead. All round him the long scarsmashed into the jungle was a bath of heat. He was clamberingheavily among the creepers and broken trunks when a bird, a vision of red and yellow, flashed upwards with a witch-like cry; and this cry was echoed by another

In just this paragraph we have a sense that the boy is in hostile terrain, not an ideal tropical paradise.

Is he sweaty because of the heat, or because he's running from something, or someone?

Is the bird really there, or is it a hallucination caused by the extreme conditions?

Just taking this first paragraph we have predominately 'scary' words. Imagine, for example if it was written like this instead:

The boy with the fair hair lowered himself down the last few feet to the beach of the lagoon. The sun shone strongly, and he'd taken his sweater off. All around him, the beach served as an artistic border, separating the water and vegetation. Continuing towards the beach, a bird flew overhead, adding its musical call to the chorus all around him.

In both cases, exactly the same events have taken place, but this should demonstrate the use of language to underline the scariness and terror of the island.

This is a great question. First, the sounds of the island are terrifying. When the fair haired boy (later known as Ralph) was climbing, he heard frightening sounds. The birds, which were red and yellow, had witch-like cries. What made this even more frightening is that these birds were also echoing each other, as if they were communicating. Here is the quote:

He was clambering heavily among the creepers and broken trunks when a bird, a vision of red and yellow, flashed upwards with a witch-like cry; and this cry was echoed by another.

The landscape was odd as well. Creepers (vines) are all over the place. They hinder movement and hold the boys down. These creepers also have thorns that prick the fat boy (later known as Piggy). Hence, the terrain can be seen as hostile and alive.

Finally, the land is also described in a way that makes it seem erie. When the fair boy finally sat down the land is said to steam. Here is the quote:

The fair boy allowed his feet to come down and sat on the steamy earth.

Based on this description, the island has every characteristic of something ominous.